Plastic optical articles are generally characterized by light weight, ease of processing and impact resistance. In the prior art, various coatings were used on such plastic optical articles for the purpose of imparting mar resistance and solvent resistance thereto. One such coating is proposed in Japanese Patent Publication No. 2735/1982 as a composition comprising an epoxy group-containing silicon compound, colloidal silica, and aluminum chelate. This composition has a problem that the coating film can be colored upon curing at high temperatures of 110.degree. C. or higher since the amount of aluminum chelate added is as large as 5 to 10% by weight based on the resin solids. In addition, boiling immersion causes the coating film to lower its hardness and sometimes peel away from the substrate.
Another coating composition is disclosed in Japanese Patent Publication No. 9266/1987 as comprising an epoxy group-containing silicon compound, colloidal silica, and ammonium perchlorate. This coating liquid is unstable and prone to a color change.
Japanese Patent Application Kokai Nos. 30361/1978 and 46502/1985 disclose the use of perchloric acid and magnesium perchlorate as the curing catalyst for similar coating compositions. The strong acidity of these catalysts causes the coating liquids to show a substantial change with the lapse of time. The silanol resulting from hydrolysis of an epoxy group-containing silicon compound can undergo poly-condensation in the presence of these acidic catalysts to soften the cured coating.
Still further coating compositions may be prepared from acidic colloidal silica and an organic silicon compound. The compositions may be cured with phosphoric acid, organic carboxylic acids, chromic acid, bromic acid, perchloric acid, aluminic acid, thiosulfuric acid or salts thereof. These coating liquids are too acidic as pH 1 or 2 and unstable.